1. Field of the Invention
The present invention pertains to the use of cascaded slurry reactors to polymerize olefins to produce polyolefin homo- and copolymers of multimodal molecular weight distribution and/or composition.
2. Background Art
Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors, and in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the loop where polymerization takes place, creating a slurry of polyethylene in solvent. A plurality of settling legs allow polymer particles to partially sediment out, creating a slurry of higher solids content, which is released periodically to harvest polymer. Slurry processes are widely used throughout the world. It has recently been proposed to cascade slurry reactors to produce multimodal resins.
Multimodal (including bidmodal) polyolefin resins are desirable due to the improved processability associated with a lower molecular weight fraction, and superior physical properties associated with a higher molecular weight component. See, e.g., U.S. Pat. No. 6,346,575 in this respect. Polyolefin resins which contain blocks of different composition, whether due to differences in properties such as long chain branching, or due to differing monomer content, are also desirable in many applications. Such polymers may be described as having a multicompositional (including bimodal, or “diblock”) monomer distribution.
Multimodal resins may be prepared by physical blending two or more resins having different molecular weight distributions. One disadvantage of such blended resins is that blending constitutes an additional process step. Moreover, the blending must be performed in such a way that a homogenous product is obtained. The blending operation not only adds additional cost to the resin, but moreover, resins produced by blending have generally inferior physicochemical properties as compared to multimodal resins having been produced by “in situ” routes.
Preparation of polymer blends in situ avoids physical blending and its disadvantages. Four types of in situ multimodal polymer production may be conceptualized. In a first process, a single reactor is employed with two distinctly different catalysts, each catalyst prepared separately on its respective support. One catalyst is selected to provide a higher molecular weight product than the other catalyst. In such a process, two distinctly different polymers are created, and the product is distinctly heterogenous. Such products are generally inferior in their processing properties, especially for applications such as film production.
In a second process, a single reactor is again used, but two different catalysts are contained on the same support, i.e., so-called “dual site” catalysts. As a result, two different polymers grow from the same catalyst particle. The resultant polymer may be described as “interstitially mixed.” A much greater degree of homogeneity in the polymer product is thus obtained at the expense of more complex catalyst preparation. Although this process offers advantages in capital and installed costs relative to multi-reactor processes, the design and synthesis of dual site catalysts is difficult. An additional process disadvantage is that use of a single reactor reduces the number of process parameters that can be manipulated to control polymer properties.
In a third process, cascaded reactors are employed, and additional catalyst is added to the second reactor. The polymer particles from the first reactor continue growth in the second reactor, although at a slower rate. However, new polymer growth begins on the newly added catalyst. Hence, as with the first process described, a heterogenous polymer product is obtained, with the same deficiencies as described previously for such products.
In a fourth process, cascaded reactors are again employed, but catalyst is added only to the first reactor. The supported catalyst associated with the first reactor polymer contain further active sites which initiate polymerization in the second reactor. The second reactor polymerization parameters are adjusted to establish a different polymerization rate and/or molecular weight range as compared to the first reactor. As a result, an interstitially mixed polymer is obtained.
EP-A-0057420 represents an example of a cascaded slurry process wherein catalyst is introduced only into the first reactor. However, molecular weight is regulated by the presence of hydrogen in both reactors, with the second reactor having higher hydrogen concentration than the first reactor, thus limiting the types of interstitially mixed polymers which may be produced. Polymerization at lower hydrogen pressure in the second reactor is not possible. In addition, the polymer formed in each reactor is limited to a specific weight percentage range relative to the weight of the final product.
U.S. Pat. No. 5,639,834 (WO 95/11930) and published application WO 95/10548 disclose use of cascaded slurry reactors in which the catalyst feed is also limited to the first reactor. In both references, the first reactor polymerization is conducted at very low hydrogen concentration, and all olefin comonomer is incorporated within the first reactor. The second polymerization is conducted at high hydrogen concentration with no comonomer feed. U.S. Pat. No. 5,639,834 additionally requires that the takeoff from the first reactor be by way of a settling leg. Continuous takeoff is said to produce inferior products. These processes do not allow operation of the second reactor at lower hydrogen concentration than the first reactor. Moreover, limiting olefin comonomer incorporation to only the first reactor limits the types of polymers which may be produced.
WO 98/58001 alleges that significant advantages in polymer properties are achievable by conducting a two-stage polymerization, the first stage at high hydrogen concentration and low comonomer concentration, and the second stage at low hydrogen concentration and high comonomer incorporation. The reactor may be a single reactor or a cascaded reactor system, the latter being preferred. A single catalyst, introduced into the first reactor, may be used. Lower hydrogen concentration in the second stage is achieved by limiting the choice of catalysts to those which rapidly consume hydrogen. Cessation of hydrogen feed thus causes the hydrogen concentration to fall rapidly between stages. The inability to add significant comonomer to the second stage or to increase comonomer incorporation in the first stage detracts from the ability to produce a wide variety of polymers. Moreover, the catalyst choice is limited to those which consume hydrogen, when a single catalyst is used.
U.S. Pat. Nos. 6,221,982 B1 and 6,291,601 B1 disclose cascaded slurry polymerizations where at least two distinct catalysts are employed. In U.S. Pat. No. 6,221,982, a Ziegler-Natta catalyst is employed in the first reactor with high hydrogen concentration and no or low comonomer incorporation. A hydrogen-consuming catalyst with low olefin polymerization efficiency is introduced downstream into the first reactor product stream. As a result, hydrogen is consumed prior to reaching the second reactor, wherein the polymerization is conducted at substantially zero hydrogen concentration. The second stage employs significant olefin comonomer. U.S. Pat. No. 6,291,601 is similar, but employs a metallocene catalyst in the first reactor.
Both the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, as well that of WO 98/58001, are inefficient in both monomer usage and thermal loading, since the hydrogenation reaction consumes ethylene, producing ethane by hydrogenation. In addition to the increased thermal loading created by this reaction, the ethane produced is an inert gas which must be purged from the system. Moreover, in the U.S. Pat. Nos. 6,221,982 and 6,291,601 processes, an additional relatively expensive hydrogenation catalyst which contributes little to polymer production must be added. Finally, all three processes require substantially homopolymerization in at least the first reactor, thus limiting the types of polymers which may be produced.
U.S. Pat. No. 6,225,421 B1 discloses use of cascaded reactors wherein ethylene is homopolymerized in the presence of hydrogen in a first reactor, hydrogen is physically separated from the first reactor product stream, and the product is copolymerized with 1-hexene and additional ethylene at reduced hydrogen concentration in the second reactor. However, the patent contains no disclosure of any apparatus suitable for removing hydrogen from the first reactor product stream. Moreover, the necessity to restrict the first polymerization to homopolymerization is limiting.
It is also desirable to produce polymers having a block configuration. One block may be different from another due to greater or lesser long chain branching, for example, or the blocks may be different due to different comonomer incorporation. In cascaded reactors, it is difficult to obtain a sharp delineation between blocks due to transfer of monomers, catalyst, etc. from the first reactor into the second. For example, if a diblock copolymer having a first block derived from copolymerizing ethylene and 1-butene and a second block derived from copolymerizing ethylene and 1-hexene, butene in the exit stream of the first reactor will cause the second block to contain butene as well as hexene.
It would be desirable to use series-configured slurry reactors wherein hydrogen is introduced into a first slurry reactor to produce a low molecular weight first polymer, following which this first polymer then introduced into a second reactor operated at lower hydrogen concentration, without the requirement of employing a catalyst which specifically encourages hydrogenation. It would further be desirable to provide a cost-effective apparatus suitable for removing hydrogen from the product stream of a first reactor operating at higher hydrogen concentration than a second reactor in series with the first. It would be yet further desirable to provide a hydrogen removal process which can accommodate comonomer incorporation in any reactor of the reactor battery. It would also be desirable to provide a process where any soluble or gaseous component can be effectively removed or reduced to low concentration prior to the entry of the first reactor product stream into a subsequent reactor, especially comonomers, such that multiblock polymers may be produced where the comonomer content of a second or subsequent block may be selected independently of the comonomer employed in the first or prior reactor.